1. Field of the Invention
This invention relates to a drive system for motor vehicles having two driven axles, which system comprises a liquid friction coupling.
2. Description of the Prior Art
In a drive system of that kind which is known from British Patent Specification No. 1,357,106 a planetary gear train comprises a planet carrier that is driven by the motor. Torque is transmitted to the two driven axles of the vehicle from the sun gear and the internal gear, respectively, of the planetary gear train, and the sun gear and internal gear are also connected to respective coupling halves of a liquid friction coupling. In that case the planetary gear train constitutes an interaxle differential, which is restrained by the liquid friction coupling. From the same printed publication it is also known to use the liquid friction coupling alone as an interaxle differential. Said two known embodiments have various disadvantages.
When the vehicle is cornering, the wheels on the two axles rotate at different speeds and the resulting speed difference between the coupling halves of the liquid friction coupling results in a restraining torque, which renders the steering of the vehicle more difficult. The steering will be particularly different at low temperatures because the restraining torque will then be increased by the high viscosity of the liquid contained in the liquid friction coupling. Besides, it is not possible to disconnect each axle from its power source although this will be required, e.g., when the vehicle is to be recovered with one axle raised with blocked wheels or when the vehicle is to be tested on test stands designed to receive only one axle. A braking system provided with an antiblock system also requires, as a rule, that the two axles can be disconnected from their comon power source. Because in the known drive system both coupling halves of the liquid friction coupling rotate, a considerable structural expenditure will be required for a control of the coupling by external means so as to modulate the interdependence of the speed difference between the coupling halves and the torque transmission capacity. Finally, in the embodiment in which the interaxle differential is constituted only by the liquid friction coupling incorporated in the drive train connected to one axle that liquid friction coupling must transmit the entire torque for driving said axle so that that coupling must have a substantial size.
From Published German Application No. 1,802,456 it is already known to associate a controllable coupling with a planetary gear train in such a manner that the coupling is coupled to a rotatable reaction member of the planetary gear train. But that known coupling is a hydraulic coupling or brake which contains a variable quantity of liquid, or the coupling consists of an eddy current coupling. Besides, the entire known arrangement does not serve to drive a vehicle but to control the speed of a machine tool, pump or the like.